You're right on the money, economic incentives drives spam in phone calls. As IoT expands, so will spam. Collectively, we can all sit behind refundable MicroPayWalls. Contacts call free. Non-contacts are ignored unless they deposit $0.05. They get a refund if the call time is more than 25s. If the call time is less than 25s, you keep their deposit. [1]

This RoboCash app does exactly that, and it pays you for each call you block as an added bonus. [2]

STIR/SHAKEN is a partial solution that doesn't go far enough to solve the problem.

[1] https://drive.google.com/file/d/1c1GyiVVZFDiUuxWkBCQ63qLdJgP...

[2] https://play.google.com/store/apps/details?id=com.fyncom.rob...

Disclaimer: I made the app and wrote the white paper.